(1) Field of the Invention
The present invention relates to high performance pneumatic radial tires which enable super high speed running.
(2) Related Art Statement
For high performance pneumatic radial tires, for example, tires have conventionally been known, which each comprise a carcass layer, a non-expandable belt layer arranged radially outside the carcass layer, a single auxiliary layer which is arranged all over the entirety of the radially outer side of the belt layer and in which organic fiber cords are buried substantially parallel to the equatorial plane of the tire, and a tread arranged radially outside the auxiliary layer. At least two circumferential main grooves are formed in the outer surface portion of the tread so that land portions are defined between the adjacent circumferential main grooves and between the axially outermost circumferential main grooves and edges of the tread.
However, although such tires can achieve running at relatively high speeds, they have a so-called chunk-out problem that when they are run at super high speeds recently required, for example, 200 to 300 km/hr, a land portion having a great mass largely swells radially outwardly due to centrifugal forces to cause blow-out. Consequently the land portion is peeled off thereby
In order to solve the above problem, a technique has been proposed, in which the swelling-out of the land portion due to the centrifugal forces is effectively prevented by constituting the auxiliary layer using two or more auxiliary plies. For example, a single auxiliary ply covering the entire width of a belt layer, and a pair of another auxiliary plies which are arranged radially outside the first auxiliary ply and which cover the entire width of the belt layer or cover axially opposite end regions of the belt layer can be used.
However, in the case of the tire of this type, since the auxiliary layer is constituted by-totally two or more auxiliary plies, the thickness of the auxiliary layer itself becomes greater. Consequently, a distance between a projection of a vulcanizing mold for the formation of the main circumferential groove and the axially outermost auxiliary ply becomes smaller at the time of termination of vulcanization. Due to this, the auxiliary layer and the belt layer is pushed and depressed radially inwardly, at locations which overlap the circumferential main grooves, by the projections of the vulcanizing mold, so that the auxiliary layer and the belt layer are entirely waved. When internal pressure is applied to the tire having the auxiliary layer and the belt layer thus waved, the auxiliary layer and the belt layer are deformed to axially expand and flatten. Such a deformation influences the shape of the surrounding land portion. That is, there is a tendency that while the central part of the land portion is depressed, the axially opposed side parts of the land portion project radially outside. As a result, since the ground contact pressure of the land portion becomes non-uniform, the tire is unevenly worn. In addition, when the entire thickness of the auxiliary layer is great as mentioned above and when the circumferential main grooves are formed in the tread rubber during vulcanizing, the distance between the bottom of the circumferential main groove and the auxiliary layer, that is, a base gauge, becomes smaller. Accordingly, it is feared that if a crack or a cut is formed on the bottom of the circumferential main groove, water or the like reaches the auxiliary layer or the belt layer, and that the auxiliary layer or the belt layer is broken.
On the other hand, in order to assure the sufficient thickness of the tread at the location corresponding to the groove, the thickness of the entire tread rubber must be made great. In that case, since the weight of the entire tread increases, this technique is disadvantageous for the high speed durability of the tire.